A new culture system has been developed for the generation and the long-term culture of pure populations of murine mast cells from marrow precursors present in both fresh marrow and long-term marrow cultures (LMC). Growth of mast cells is dependent upon the continuous presence of mast cell growth factor (MCGF) contained in media conditioned by mitogen activated splenic leukocytes or murine lymphoma cell line LBRM-33. As a variety of hemopoietically active factors are present in these conditioned media, the proposed project will encompass a series of experiments to analyze factor production by LBRM-33 cells, as well as other in vitro adapted isolates of radiation induced lymphomas of the LBRM series. Procedures are proposed for the fractionation and purification of these factors from the conditioned media. The variety of in vitro cloning and liquid culture techniques will be applied to aid in the characterization of hemopoietic precursors by serologic means, and by velocity sedimentation. Particular reference will be made toward resolving the controversy over the origin of mast cells and the question of the immune responsiveness of T cell precursors in LMC. In addition, pure populations of mast cells will be used to study the function of mast cells, their differentiation, maturation, as well as interaction with other cells of the hemopoietic system. Finally, the long-term mast culture system will be applied to analyze the cause of mast cell deficiency in W and S1 anemic mice.